There are high unmet medical needs in the few established therapies for several cardiovascular and cerebrovascular diseases, autoimmune diseases and inflammatory disorders, fibrotic diseases, metabolic diseases and oncologic diseases. Despite the diverse clinical manifestations of these diseases, they share a unique common disease pathogenesis characterized by organ and tissue damage arising from dysregulated immune responses and production of critical inflammatory mediators. Only recently has the overlapping mechanistic role of MIF been well-characterized and target validated in several animal models of some of these diseases.
MIF is a pro-inflammatory cytokine secreted by activated T cells and macrophages that critically regulates inflammation. MIF plays an important role in the host innate and adaptive immune responses through its direct biological function and also through downstream signaling events following its binding with its known receptors CD74, CXCR2 and CXCR4 (Greven et al., 2010; Morand et al., 2006; Calandra and Roger, 2003; Gore et al., 2008; Bernhagen et al., 2007; Cho et al., 2010; McLean et al., 2010; Weber et al., 2008). MIF is an important mediator in the initiation and perpetuation of the inflammatory process through T-cell proliferation, B-cell antibody production, macrophage activation and induction of inflammatory mediators, as well as cell growth promotion, angiogenesis and counter-regulation of glucocorticoids contributing to disease progression. MIF has been implicated in the pathogenesis of a wide range of disorders including cardiovascular and cerebrovascular diseases, autoimmune and inflammatory diseases, fibrotic diseases, metabolic diseases, and oncologic diseases. Thus, MIF is a therapeutic target for many diseases and disorders.
Among cytokines, MIF is unique because it functions as an enzyme exhibiting tautomerase catalytic activity which was initially thought to underlie MIF's biologic function. The tautomerase enzyme catalytic activity site is located within the canonical deep pocket of MIF. As such, most first generation MIF inhibitors selectively target this MIF catalytic activity site. It has recently been demonstrated that inhibition of MIF tautomerase activity is not tantamount to complete inhibition of MIF biological properties (Fingerle-Rowson et al., 2009). In particular, a tautomerase-null, Pro->Gly1 MIF protein (P1G-MIF) knock-in mouse model showed in this study that intrinsic tautomerase enzyme activity is dispensable for MIF's biological properties. Catalytically inactive P1G-MIF shows preservation, albeit attenuated, of MIF biological functions and of CD74- and CXCR2-binding, supporting the important role for other specific residues and motifs in MIF within and outside the catalytic site that regulates function and receptor interactions.
Recent advances in the structural biology and chemistry of MIF have revealed critical pharmacophores in addition to those in the MIF tautomerase catalytic site that should serve as important targets for the development of MIF inhibitors which display better target binding specificity and enhanced therapeutic efficacy against MIF-related diseases (Greven et al., 2010; Morand et al., 2006; Calandra and Roger, 2003; Gore et al., 2008; Bernhagen et al., 2007; Cho et al., 2010; McLean et al., 2010). For example, a new allosteric surface binding pocket has been discovered at the mouth of the canonical deep pocket catalytic site that contains specific residues important for MIF conformational changes and receptor binding (McLean et al., 2010; Cho et al., 2010). These specific MIF residues within the canonical deep pocket and the surface allosteric binding site of MIF have been identified to be important contact sites for CD74 and CXCR2 receptor binding which mediate critical MIF signal transduction activity.
Recent advances in understanding the complex biology of MIF have demonstrated MIF functioning not only through interaction with CD74, but also through CXCR2 and CXCR4 receptors (Bernhagen et al., 2007; Weber et al., 2008). Furthermore, specific residues on MIF have been described as critical for interaction with CD74, CXCR2, and CXCR4 receptors and these have not been concomitantly targeted by first generation MIF tautomerase inhibitors (Cournia et al., 2009; Weber et al., 2008; McLean et al., 2010). Lack of inhibition by these first generation MIF inhibitors of MIF/receptor binding and thus numerous MIF-induced downstream signal transduction events have limited their therapeutic potential in MIF-related diseases. Most of the first generation MIF inhibitors do not reflect the scientific developments on MIF biology and chemistry which have recently emerged. These first generation MIF inhibitors mostly targeted the MIF enzymatic tautomerase activity located at the canonical deep pocket site around the N-terminal Pro-1 region and do not target either the surface allosteric site or the specific MIF residues within both binding sites critical for receptor interactions. Such a catalytic site-specific approach utilized by the first generation MIF inhibitors may be the reason that certain MIF biological functions and MIF-mediated signal transduction events remain partially uninhibited in the presence of these inhibitors due to insufficient conformational change of MIF and incomplete inhibition of MIF:receptor binding. In contrast, inhibitors which interact with a combination of allosteric and catalytic sites will induce a conformational change and block both tautomerase activity and critical MIF:receptor interactions resulting in effective functional antagonism and blockade of downstream signaling.
Thus, new inhibitors of MIF are currently needed for use in treating MIF associated diseases and disorders.